Minimal Thickness Synthetic Antiferromagnetic (SAF) Structure with Perpendicular Magnetic Anisotropy for STT-MRAM

ABSTRACT

A synthetic antiferromagnetic (SAF) structure for a spintronic device is disclosed and has an FL2/AF coupling/CoFeB configuration where FL2 is a ferromagnetic free layer with intrinsic PMA. In one embodiment, AF coupling is improved by inserting a Co dusting layer on top and bottom surfaces of a Ru AF coupling layer. The FL2 layer may be a L10 ordered alloy, a rare earth-transition metal alloy, or an (A1/A2) n  laminate where A1 is one of Co, CoFe, or an alloy thereof, and A2 is one of Pt, Pd, Rh, Ru, Ir, Mg, Mo, Os, Si, V, Ni, NiCo, and NiFe, or A1 is Fe and A2 is V. A method is also provided for forming the SAF structure.

This is a Divisional application of U.S. patent application Ser. No.13/609,780, filed on Sep. 11, 2012, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

RELATED PATENT APPLICATION

This application is related to U.S. Pat. No. 8,064,244 which is hereinincorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a Magnetoresistive Random AccessMemory (MRAM) that includes a magnetic tunnel junction (MTJ) in which acomposite reference layer has antiferromagnetic coupling between a layerwith intrinsic perpendicular magnetic anisotropy (PMA) and a CoFeB layerto establish PMA in the latter, and composite reference layer thicknessis reduced to minimize the stray field (Ho) on the free layer.

BACKGROUND

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with MTJ technology, is a major emerging technology thatis highly competitive with existing semiconductor memories such as SRAM,DRAM, and Flash. Similarly, spin-transfer (spin torque or STT)magnetization switching described by C. Slonczewski in “Current drivenexcitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7(1996), has recently stimulated considerable interest due to itspotential application for spintronic devices such as STT-MRAM on agigabit scale.

Both MRAM and STT-MRAM have a MTJ element based on a tunnelingmagneto-resistance (TMR) effect wherein a stack of layers has aconfiguration in which two ferromagnetic layers are separated by a thinnon-magnetic dielectric layer, or based on a GMR effect where areference layer and free layer are separated by a metal spacer. The MTJelement is typically formed between a bottom electrode such as a firstconductive line and a top electrode which is a second conductive line atlocations where the top electrode crosses over the bottom electrode. AMTJ stack of layers may have a bottom spin valve configuration in whicha seed layer, reference layer, a thin tunnel barrier layer, aferromagnetic “free” layer, and a capping layer are sequentially formedon a bottom electrode. The reference layer has a fixed magnetizationdirection. The free layer has a magnetic moment that is either parallelor anti-parallel to the magnetic moment in the reference layer. Thetunnel barrier layer is thin enough that a current through it can beestablished by quantum mechanical tunneling of conduction electrons.When a sense current is passed from the top electrode to the bottomelectrode in a direction perpendicular to the MTJ layers, a lowerresistance is detected when the magnetization directions of the free andreference layers are in a parallel state (“0” or P memory state) and ahigher resistance is noted when they are in an anti-parallel state (“1”or AP memory state). In STT-MRAM, the resistance can be switched betweenthe two states by the application of a current pulse of sufficientmagnitude to write the bit to the opposite state.

As the size of MRAM cells decreases, the use of external magnetic fieldsgenerated by current carrying lines to switch the magnetic momentdirection becomes problematic. One of the keys to manufacturability ofultra-high density MRAMs is to provide a robust magnetic switchingmargin by eliminating the half-select disturb issue. Compared withconventional MRAM, spin-transfer torque or STT-MRAM has an advantage inavoiding the half select problem and writing disturbance betweenadjacent cells. The spin-transfer effect arises from the spin dependentelectron transport properties of ferromagnetic-spacer-ferromagneticmultilayers. When a spin-polarized current transverses a magneticmultilayer in a CPP configuration, the spin angular moment of electronsincident on a ferromagnetic layer interacts with magnetic moments of theferromagnetic layer near the interface between the ferromagnetic andnon-magnetic spacer. Through this interaction, the electrons transfer aportion of their angular momentum to the ferromagnetic layer. As aresult, spin-polarized current can switch the magnetization direction ofthe ferromagnetic layer if the current density is sufficiently high, andif the dimensions of the multilayer are small. The difference between aSTT-MRAM and a conventional MRAM is only in the write operationmechanism. The read mechanism is the same.

Typically, the magnetic moments of the reference layer and free layerare in an in-plane direction. However, for a variety of reasons, it isadvantageous to engineer perpendicular magnetic anisotropy (PMA) intothe aforementioned layers so that their magnetization direction isperpendicular-to-plane. The source of PMA may be intrinsic or it may beinduced in a ferromagnetic layer at interfaces with an oxide layer, forexample, in situations where the ferromagnetic layer has a thicknessless than a threshold level. A viable PMA bit needs to exhibit PMA inboth free and reference layers in order to generate tunnelingmagnetoresistance (TMR). Spintronic devices with perpendicular magneticanisotropy have an advantage over MRAM devices based on in-planeanisotropy in that they can satisfy the thermal stability requirementbut also have no limit of cell aspect ratio. As a result, spin valvestructures based on PMA are capable of scaling for higher packingdensity which is one of the key challenges for future MRAM applicationsand other spintronic devices.

In a MTJ within a MRAM or STT-MRAM, a reference layer will usually exerta stray magnetic field upon the free layer that tends to favor eitherthe P or AP state. The stray field (Ho) has a form similar to anon-uniform electric “fringing” field at the edges of a parallel platecapacitor. As depicted in FIG. 1, the stray field Ho 4 from referencelayer 1 impinges on the free layer 3. Note that a dielectric spacer 2such as a tunnel barrier layer separates the free layer and referencelayer. When the reference layer 1 is a composite, the net stray field 4will be the sum of fringing fields from several similar layers in thereference layer stack with the possible addition of a uniform effective“interlayer” coupling field. The free layer is subject to random thermalagitation and the stray field Ho will create a disparity in the thermalstability of the two states, with either the P or AP state rendered morethermally stable. This asymmetry is undesirable since for a given freelayer coercivity (Hc), Ho should be zero for optimum stability.Generally, Ho=0 is difficult to achieve in practice, and as a rule,Ho<15% of Hc is a reasonable target in actual devices.

Referring to FIG. 2, a synthetic antiferromagnetic (SAF) structure 18 iscommonly employed as a reference layer to reduce the magnitude of Hothat impinges on a free layer 17. The SAF stack consists of twoferromagnetic layers labeled AP2 11, and AP1 13 which are coupledantiferromagnetically through an intervening non-magnetic layer 12 thatis typically Ru. There is a tunnel barrier layer 16 formed between theSAF structure 18 and free layer 17.

The net stray magnetic field Ho exerted by SAF structure 18 in aSTT-MRAM bit with a 40 nm diameter is usually more than 500 Oe which isan unacceptably high value of about the same magnitude as the free layerHc. Therefore, an improved reference layer is needed that generates asufficiently small Ho to avoid disrupting the P or AP state in a freelayer within the MTJ.

SUMMARY OF THE INVENTION

One objective of the present disclosure is to provide a syntheticantiferromagnetic (SAF) structure for a STT-MRAM which has PMA and a Hofield substantially less than that produced by a SAF layer with anAP2/Ru/AP1/CoFeB configuration.

A second objective of the present disclosure is to provide a SAFstructure according to the first objective that may serve as a referencelayer and/or a free layer in a magnetic tunnel junction (MTJ).

According to one embodiment, these objectives are achieved in a MTJhaving a bottom spin valve configuration for spintronic deviceapplications such as a read/write head, or spin-transfer oscillatordevices for MRAM, or microwave assisted memory recording (MAMR). The MTJis comprised of a stack of layers including a composite seed layer, aSAF reference layer, tunnel barrier layer, free layer, and cap layerwhich are sequentially formed on a substrate. The seed layer may beTaN/Mg/NiCr, for example, and is critical for enhancing the (111)texture and PMA character in overlying layers. In one aspect, the SAFreference layer has an AP2/Ru/CoFeB configuration where the AP2 layerhas intrinsic PMA and is made of a laminated stack (A1/A2)_(n) where A1is one of Co and CoFe or an alloy thereof, and A2 is one of Pt, Pd, Rh,Ru, Ir, Mg, Mo, Os, Si V, Ni, NiCo, and NiFe where the number oflaminates “n” is between 1 and 10, and preferably less than 6.Optionally, A1 is Fe and A2 is V. The overall thickness of the SAFstructure is minimized by reducing the thickness of the AP2 and CoFeBlayers in order to reduce Ho. Furthermore, as the number of laminates isdecreased, coercivity (Hc) increases. As the CoFeB thickness decreasesto less than 10 Angstroms, PMA is induced in the CoFeB layer throughantiferromagnetic (AF) coupling with the laminated AP2 stack.

Alternatively, the laminated AP2 structure may be replaced by a L10ordered alloy of the form MT wherein M is Rh, Pd, Pt, Ir, or an alloythereof, and T is Fe, Co, Ni or alloy thereof. Furthermore, the MT alloymay be doped with B to give a boron content up to 40 atomic %. In yetanother embodiment, the AP2 layer may be an amorphous rareearth-transition (RE-TM) alloy such as TbCo, TbFeCo, or GdFeCo thatexhibits PMA. A tunnel barrier comprised of MgO or another metal oxideformed on a top surface of the SAF structure helps to maintain PMAcharacter within the thin CoFeB layer. There is a free layer contactinga top surface of the tunnel barrier and a cap layer as the uppermostlayer in the MTJ stack. Preferably, the free layer has PMA to enhancethe TMR ratio in the MTJ.

In a second embodiment, a top spin valve structure is provided wherein aseed layer, SAF free layer, tunnel barrier, reference layer, and cappinglayer are sequentially formed on a substrate. The SAF free layer iscomprised of a FL2/Ru/CoFeB configuration where the FL2 layer hasintrinsic PMA and contacts a top surface of the seed layer. The FL2layer is comprised of a laminated structure, a L10 alloy, or a rareearth alloy as described previously with respect to the AP2 layer in thefirst embodiment. The CoFeB layer is preferably less than 10 Angstromsthick and has a top surface that adjoins a tunnel barrier made of MgO orthe like. The reference layer has intrinsic PMA and may be made of thesame material as the AP2 layer in the first embodiment. In bothembodiments, the Ru coupling layer is preferably 4 or 9 Angstroms thickto provide optimum antiferromagnetic coupling between the FL2 (or AP2)layer and the CoFeB layer. In alternative embodiments, the SAF freelayer may be formed in a bottom spin valve configuration and the SAFreference layer may be employed in a top spin valve configuration.Moreover, both of the SAF reference layer and SAF free layer describedherein may be combined in the same MTJ stack of layers.

In another embodiment, the antiferromagnetic coupling is enhanced byinserting a Co dusting layer along each of the top and bottom surfacesof the Ru coupling layer. Thus, the SAF structure has a stackrepresented by AP2/Co/Ru/Co/CoFeB for a reference layer in a bottom spinvalve configuration and FL2/Co/Ru/Co/CoFeB for a free layer in a topspin valve configuration.

In all embodiments, once all of the layers in the MTJ stack are laiddown, a patterning and etching sequence is followed to fabricate a spinvalve structure that may be in the shape of an oval, circle, or polygonfrom a top-down view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a MTJ wherein the stray fringe fieldHo from the reference layer is exerted on a free layer.

FIG. 2 is a cross-sectional view of a MTJ with a bottom spin valveconfiguration wherein a reference layer has a SAF structure representedby AP2/coupling layer/AP1.

FIG. 3 is cross-sectional view of a MTJ with a bottom spin valveconfiguration wherein the reference layer is a composite with aAP2/Ru/AP1/CoFeB stack and the AP1 layer is ferromagnetically coupledwith the CoFeB layer.

FIG. 4 is a cross-sectional view of a MTJ with a bottom spin valveconfiguration in which the reference layer is a composite with an AP2layer having intrinsic PMA and AF coupled to a CoFeB layer according toan embodiment of the present disclosure.

FIG. 5 is a cross-sectional view of a MTJ with a bottom spin valveconfiguration similar to that depicted in FIG. 3 except a Co dustinglayer is inserted on either side of the Ru coupling layer according to asecond embodiment of the present disclosure.

FIG. 6 is a cross-sectional view of a MTJ with a top spin valveconfiguration wherein a FL2 free layer with intrinsic PMA isantiferromagnetically coupled to a CoFeB layer according to a secondembodiment of the present disclosure.

FIG. 7 is a cross-sectional view of a MTJ with a bottom spin valveconfiguration wherein the free layer has a SAF structure with a CoFeBlayer contacting a top surface of the tunnel barrier layer.

FIG. 8 is a cross-sectional view of a MTJ with a top spin valveconfiguration wherein the reference layer has a SAF structure with aCoFeB layer contacting a top surface of the tunnel barrier layer.

FIG. 9 is a cross-sectional view of a MTJ with a bottom spin valveconfiguration that employs both of a SAF free layer and SAF referencelayer structure wherein both SAF structures contain Co dusting layersaccording to an embodiment of the present disclosure.

FIG. 10 is modification of the MTJ in FIG. 5 where an insertion layer isformed between the AF coupling layer and CoFeB layer in an SAFstructure.

FIG. 11 is a plot of Kerr signal as a function of applied field for areference layer stack made of (Co/Ni)₁₀/Ru/CoFeB and with variousthickness for the CoFeB layer.

FIG. 12 is a plot of Kerr signal as a function of applied field for areference layer stack made of (Co/Ni)_(n)/Ru/CoFeB wherein the number oflaminates is varied while keeping a constant CoFeB thickness.

FIGS. 13 a, 13 b are plots that show change in loop shift with junctionresistance for a prior art MTJ and a MTJ having a SAF reference layerformed according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure is a SAF structure which includes a layer withintrinsic PMA that induces PMA in a thin CoFeB layer across anantiferromagnetic coupling layer for enhanced performance in spintransfer oscillators including MAMR devices, STT-MRAM devices, and inother spintronic devices. When the intrinsic PMA layer is made of alaminated stack (A1/A2)_(n) where A1 is one of Co and CoFe or an alloythereof, and A2 is one of Pt, Pd, Rh, Ru, Ir, Mg, Mo, Si, Os, V, Ni,NiCo, and NiFe, then A1 and A2 may be switched to give a (A2/A1)_(n)stack providing the same advantages as the (A1/A2)_(n) arrangement.Although only bottom spin valve or top spin valve configurations areshown in the drawings, the present disclosure also encompasses dual spinvalves as appreciated by those skilled in the art.

As mentioned in related U.S. Pat. No. 8,064,244, the magnetic anisotropyof a (Co/Ni)_(n) laminated structure arises from the spin-orbitinteractions of the 3d and 4s electrons of Co and Ni atoms. Suchinteraction causes the existence of an orbital moment which isanisotropic with respect to the crystal axes which are in (111)alignment, and also leads to an alignment of the spin moment with theorbital moment. In (Co/Ni)_(n) laminates and the like as represented by(A1/A2)_(n), it is essential to have a fcc (111) super-lattice in orderto establish PMA. Previously, the inventors have employed a referencelayer consisting of a single laminated stack of (Co/Ni)_(n) as describedin U.S. Pat. No. 8,064,244, or a synthetic antiferromagnetic (SAF)structure represented by AP2/Ru/AP1/CoFeB. However, the Ho fieldassociated with both of these schemes is too high and leads to decreasedthermal stability in the free layer.

Referring to FIG. 3, the SAF reference layer 15 having intrinsic PMA andwith a AP2/Ru/AP1/CoFeB configuration is further described in that AP1and AP2 are both bulk-PMA multilayers made of (Co/Pd)_(n), (Co/Pt)_(n),(Co/Ni)_(n) or the like where n is the number of laminates, or an L10alloy such as FePt or FePd, or an amorphous rare earth-transition metalalloy such as TbCo. A thin CoFeB dusting layer 14 is formed adjacent toa tunnel barrier 16 and is ferromagnetically coupled to the AP1 layer.The dusting layer promotes substantially higher TMR than the AP1 layerby itself since the amorphous CoFeB crystallizes to a bcc structureduring post-deposition annealing thereby increasing TMR especially whenadjoining a (100) MgO tunnel barrier layer. A CoFeB thickness isselected that is a compromise between a thick layer for higher TMR and athin layer that is below the threshold level to establish PMA andprovide high coercivity where SAF Hc>free layer Hc. As a result, highcoercivity and PMA are provided by AP2 layer 11 and AP1 layer 13 whilethe thin dusting layer is responsible for good electrical properties(high TMR). AP1 layer 13 and CoFeB dusting layer 14 areferromagnetically coupled and have a magnetization in the same directionthat is perpendicular to the planes of the layers. Without the PMAinfluence provided by AP1 and AP2, the magnetic moment of the dustinglayer would be in-plane and result in zero TMR between P and AP states.

It is also important that a SAF structure be “balanced” which means thesaturation magnetization x thickness product (Mst) ratio between thelayers on either side of the Ru interlayer should be approximately 1.00.In SAF reference layer 15, when AP1 is (Co/Ni)₁₀, AP2 is (Co/Ni)₆, eachof the Co layers has a 2.5 Angstrom thickness, each of the Ni layers hasa 6 Angstrom thickness, and CoFeB thickness is 10 Angstroms, then (MstAP1+Mst CoFeB)/Mst AP2=1.00. Unfortunately, the stray field Ho generatedby this SAF structure is over 500 Oe which is an unacceptably high valuefor STT-MRAM bits.

We have surprisingly found that decreasing the thickness of the SAFstructure in FIG. 3 leads to a substantial reduction in the Ho fieldexerted by a composite reference layer on the free layer. Meanwhile, asignificant Hc is maintained within the reference layer and a balancedMst ratio is achieved when the reduction of SAF structure thickness isaccomplished by a combination of modifications including removal of theAP1 layer, and preferably thinning both of the AP2 layer and CoFeBlayer. As a result, PMA character is conveyed to the CoFeB layer not bydirect ferromagnetic coupling to the AP1 layer as in the aforementionedFIG. 3 scheme, but through antiferromagnetic coupling with the AP2layer.

According to one embodiment as depicted in FIG. 4, the SAF structure isemployed as a composite reference layer in a bottom spin valveconfiguration wherein a MTJ is comprised of a seed layer 21, SAFreference layer 28, tunnel barrier layer 25, free layer 26, and caplayer 27 that are sequentially formed on a substrate 20. The substratemay be a bottom electrode in a STT-MRAM device. The seed layer 21 may bea composite such Ta/M1/M2 where M1 is a metal having a fcc(111) or (hcp)hexagonal closed packed (001) crystal orientation such as Ru, and M2 isCu, Ti, Pd, Pt, W, Rh, Au, or Ag. In another preferred embodiment, theseed layer may have a TaN/Mg/X, Ta/X, or Ta/Mg/X configuration wherein Xis a growth enhancement layer and is one of NiCr or NiFeCr. However,other seed layer materials are acceptable that promote a (111) crystalorientation in an overlying SAF reference layer.

A key feature of the present disclosure is the SAF reference layer 28that includes a lower AP2 (ferromagnetic) layer 22 having intrinsic PMA,a middle antiferromagnetic coupling layer 23, and an upper CoFeB layer24. In one aspect, the AP2 layer is a (A1/A2)_(n) laminated stack whereA1 is one of Co and CoFe or an alloy thereof, and A2 is one of Pt, Pd,Rh, Ru, Ir, Mg, Mo, Os, Si, V, Ni, NiCo, and NiFe where the number oflaminates “n” is between 1 and 10, and preferably less than 6 tominimize the thickness and Ho field of the SAF structure whilemaintaining high coercivity (Hc) and sufficient PMA to support PMA inthe CoFeB layer 24. Each of the A1 layers has a thickness from 0.5 to 5Angstroms, and preferably between 1.5 to 3 Angstroms. Each of the A2layers in the laminated stack has a thickness from 2 to 10 Angstroms,and preferably between 3.5 and 8 Angstroms. Preferably, the thickness t2of an A2 layer is greater than an A1 layer thickness t1, and morepreferably, t2˜2X t1 in order to optimize the spin orbit interactionsbetween adjacent A1 and A2 layers. It should be understood that when A2is one of Rh, Ir, Ru, Os, Mo, or an alloy thereof, and n is between 2and 10, ferromagnetic or antiferromagnetic coupling is establishedbetween neighboring A1 layers in the (A1/A2)_(n) laminate. In anotherembodiment, A1 is Fe and A2 is V.

In an alternative embodiment, the AP2 layer 22 may be a L10 orderedalloy of the form MT wherein M is Rh, Pd, Pt, Ir, or an alloy thereof,and T is Fe, Co, Ni or alloy thereof. Furthermore, the MT alloy may bedoped with B to give a boron content up to 40 atomic %. In yet anotherembodiment, the AP2 layer may be a rare earth-transition metal alloysuch as TbCo, TbFeCo, or GdFeCo. The antiferromagnetic coupling layer 23is non-magnetic and preferably is one of Ru, Ir, Rh, Os, Mo, V, or analloy thereof. When Ru is selected as the coupling layer, the thicknessis preferably 4 or 9 Angstroms to provide maximum coupling strengthbetween the AP2 layer 22 and CoFeB layer 24.

In order to induce PMA character in the CoFeB layer 24 throughantiferromagnetic coupling with AP2 layer 22, and maintain the PMAthroughout the device lifetime, the CoFeB layer preferably has athickness less than about 12 Angstroms. Furthermore, the CoFeB layerthickness should be at least 6 Angstroms to promote a high TMR ratio inthe MTJ. It is believed that the tunnel barrier layer also induces acertain degree of PMA in the CoFeB layer along the interface betweenlayers 24, 25.

In a second embodiment illustrated in FIG. 5, antiferromagnetic couplingbetween AP2 layer 22 and CoFeB layer 24 may be enhanced by furtherincluding a Co dusting layer that adjoins both of the top and bottomsurfaces of a Ru coupling layer, for example. A Co rich alloy with a Cocontent above 50 atomic % may also be used as a dusting layer.Preferably, the Co or Co alloy dusting layers 29 a, 29 b have athickness from about 2 to 10 Angstroms, and more preferably about 4Angstroms. A first Co dusting layer 29 a contacts a top surface of AP2layer 22 and a bottom surface of the coupling layer 23. A second Codusting layer 29 b contacts a top surface of the coupling layer 23 and abottom surface of the CoFeB layer 24. In this embodiment, the combinedthickness of second dusting layer 29 b and CoFeB layer 24 is preferablyfrom 7 to 15 Angstroms.

The tunnel barrier layer 25 is comprised of MgO or another metal oxidesuch as AlOx, TiOx, and ZnOx. A MgO tunnel barrier layer may befabricated by depositing a first Mg layer on the CoFeB layer 24, andthen performing a natural oxidation (NOX) or radical oxidation (ROX)process, and finally depositing a second Mg layer on the oxidized firstMg layer. During a subsequent annealing process, the second Mg layer isoxidized to afford a substantially uniform MgO layer. Typically forSTT-MRAM, a ROX process is preferred for tunnel barrier formation inorder to produce a relatively high resistance x area (RA) value of up to1000 ohm-um² or more. However, the present disclosure anticipates that aNOX method may be used to make the tunnel barrier layer 25.

The free layer 26 may be a thin CoFeB layer wherein PMA is inducedthrough an interface with the tunnel barrier layer. Furthermore, a PMAenhancing layer such as MgO or another metal oxide may be insertedbetween the free layer 26 and cap layer 27 to further induce PMA in thefree layer through a second interface. In another embodiment, a materialwith intrinsic PMA such as a laminated (A1/A2)_(n) stack, L10 alloy, orRE-TM alloy described with respect to AP2 layer 22 may be employed asthe free layer. The present disclosure also anticipates the free layer26 may have a SAF structure represented by CoFeB/Co/Ru/Co/FL2 in abottom spin valve configuration where FL2 is (A1/A2)_(n), the Co layersadjoining the Ru AF coupling layer enhance the AF coupling between FL2and CoFeB, and the CoFeB portion of the free layer contacts a topsurface of the tunnel barrier layer. In the aforementioned SAF freelayer structure, the combined thickness of CoFeB and adjoining Codusting layer is from 7 to 15 Angstroms.

Preferably, the cap layer 27 is a material that provides good electricalcontact with an overlying top electrode (not shown), and getters oxygenfrom the free layer to improve the TMR ratio. Examples of cap layers areRu/Ta, Ta/Ru, and Ru/Ta/Ru, although other cap layer materials used inthe industry may be selected for the MTJ of the present disclosure. Inan embodiment wherein both of the reference layer and free layer have aSAF structure as described above, the cap layer may be NiCr/Ru or Ni/Tato support PMA in the FL2 layer that contacts the cap layer. Thus, thepresent disclosure encompasses a MTJ stack represented byTaN/Mg/NiCr/AP2/Co/Ru/Co/CoFeB/MgO/CoFeB/Co/Ru/Co/FL2/NiCr/Ru whereTaN/Mg/NiCr is a seed layer for the SAF reference structure and NiCr/Ruis a cap layer on the SAF free layer structure.

In FIG. 6, a third embodiment of the present disclosure is describedwherein the SAF structure previously described is used as a free layerin a MTJ with a top spin valve configuration. All of the layers areretained from FIG. 4 except for free layer 26 which is now SAF freelayer 32 that is comprised of a lower FL2 (ferromagnetic) layer 31having intrinsic PMA and contacting a top surface of seed layer 21, amiddle antiferromagnetic coupling layer 23, and upper CoFeB layer 24. Inother words, seed layer 21, SAF free layer 32, tunnel barrier 25,reference layer 22, and capping layer 27 are sequentially formed onsubstrate 20. Reference layer 22 has intrinsic PMA and is comprised ofan (A1/A2)_(n) laminated stack, an L10 alloy, or a RE-TM alloy asdescribed earlier. Likewise, FL2 layer 31 has intrinsic PMA and ispreferably selected from an (A1/A2)_(n) laminated stack, an L10 alloy,or a RE-TM alloy as described previously. The present disclosure alsoanticipates that reference layer 22 with intrinsic PMA may be replacedby a reference layer comprised of a thin CoFeB layer from 6 to 15Angstroms thick wherein PMA is induced in the CoFeB layer through aninterface with the tunnel barrier layer 25.

There are multiple advantages of a free layer having a SAF structureaccording to the third embodiment wherein PMA is induced in a CoFeBlayer through antiferromagnetic coupling with an intrinsic PMA layer.First, the effect of stray fringing fields (Ho) from the reference layeris minimized. In addition, the CoFeB layer 24 with PMA enables a highTMR ratio with high thermal stability. Furthermore, the free layermaintains high intrinsic and adjustable coercivity.

Referring to FIG. 7, the present disclosure also anticipates anembodiment wherein the SAF free layer structure in FIG. 6 is furthercomprised of a first Co dusting layer 29 a formed between FL2 layer 31and the coupling layer 23, and a second Co dusting layer 29 b insertedbetween the coupling layer 23 and CoFeB layer 24 to strengthen theantiferromagnetic coupling between the FL2 layer and CoFeB layer. Themodified SAF free layer structure 36 contacts a top surface of seedlayer 21. According to one embodiment, tunnel barrier layer 25,reference layer 22, and cap layer 27 are formed in consecutive order onCoFeB layer 24. However, one skilled in the art will appreciate thatreference layer 22 may be substituted with a SAF reference layerstructure wherein a CoFeB layer, an antiferromagnetic coupling layer,and a reference layer with intrinsic PMA are sequentially formed on thetunnel barrier layer.

Referring to FIG. 8, the present disclosure also encompasses anembodiment wherein the MTJ has a bottom spin valve configuration andboth of the reference layer and free layer have a SAF structure. In theexemplary embodiment, a MTJ is shown wherein a seed layer 21, SAFreference layer 28, and tunnel barrier 25 are sequentially formed on asubstrate as in the first embodiment. The SAF reference layer has alower AP2 layer with intrinsic PMA, a middle antiferromagnetic couplinglayer 23 a, and an upper CoFeB layer 24 a. In this case, a SAF freelayer 38 consisting of a lower CoFeB layer 24 b, middleantiferromagnetic coupling layer 23 b, and FL2 layer 31 with intrinsicPMA is formed on a top surface of the tunnel barrier. The uppermostlayer in the MTJ is a cap layer that contacts a top surface of FL2 layer31. Layers 23 a, 23 b have the same thickness and properties aspreviously described for antiferromagnetic coupling layer 23. CoFeBlayers 24 a, 24 b preferably have a thickness between 6 and 12 Angstromssuch that PMA may be induced and maintained within the CoFeB layersthrough antiferromagnetic coupling with the AP2 and FL2 layers,respectively, and in part through the interface with the tunnel barrierlayer 25.

In an alternative embodiment depicted in FIG. 9 where a first Co dustinglayer 29 a is inserted between layers 23 a and 24 a, and a second Codusting layer 29 b is inserted between layers 23 b and 24 b, then thecombined thickness of first Co dusting layer and CoFeB layer 24 a isfrom 7 to 15 Angstroms. Likewise, the combined thickness of the secondCo dusting layer and CoFeB layer 24 b is preferably between 7 and 15Angstroms. Moreover, there may be a third Co dusting layer 29 c betweenAP2 layer 22 and coupling layer 23 a, and a fourth Co dusting layer 29 dbetween FL2 layer 31 and coupling layer 24 b. Thus, this embodimentfeatures a SAF free layer structure 40 and a SAF reference layerstructure 30 both having Co dusting layers adjoining AF coupling layers23 a, 23 b.

Referring to FIG. 10, another embodiment is depicted wherein the SAFstructure 30 in FIG. 5 is modified to form SAF structure 39 thatincludes a non-magnetic insertion layer 33 made of Ta, Al, Cu, Zr, Hf,Nb, Mg, or Mo formed between the dusting layer 29 b and CoFeB layer 24.The insertion layer serves as a moment dilution layer and has athickness from 0.5 to 10 Angstroms, and preferably between 1 and 5Angstroms.

Note that in all of the aforementioned embodiments, the CoFeB layer inthe SAF structure contacts the tunnel barrier layer. In other words,when a SAF free layer or SAF reference layer structure is formed betweenthe substrate and tunnel barrier, the CoFeB layer is the uppermost layerin the SAF stack. However, when the SAF structure is formed between thetunnel barrier and cap layer, then the CoFeB layer is below theantiferromagnetic coupling layer and AP2 (or FL2) layer and is thebottom layer in the SAF stack.

After all layers in the full field MTJ stack are laid down, the stackmay be processed as deposited or may be annealed at temperatures between200° C. and 500° C. in embodiments where Co or Co alloy dusting layersare included to enhance AF coupling between the AP2 (or FL2) layer andCoFeB layer in one or both of a SAF reference layer structure and SAFfree layer structure. For embodiments where Co or Co alloy dustinglayers are omitted, the upper limit for annealing temperature ispreferably 350° C.

Example 1

To further describe the effect of AP2 layer thickness on SAF referencelayer properties with regard to the second embodiment that has anAP2/Co/Ru/Co/CoFeB configuration, a MTJ stack was fabricated with thefollowing bottom spin valve configuration where the number following thelayer indicates the layer thickness:TaN20/Mg7/NiCr50/(Co2.5/Ni6)₁₀/Co4/Ru4/Co4/Co₂₀Fe₆₀B₂₀10/MgO(8/4ROX)/Co₂₀Fe₆₀B₂₀3/Ta20/Ru50.In this case, TaN/Mg/NiCr is the seed layer and Ta/Ru serves as a caplayer. The MgO tunnel barrier is made by first depositing an 8 Angstromthick Mg layer, followed by an ROX process, and then depositing a secondMg layer that is 4 Angstroms thick. Ho is measured to be −190 Oe and theMst balance ratio=0.40. If the number of laminates “n” in the(Co/Ni)_(n) AP2 stack is reduced from 10 to 6, then Ho=0 and the balanceratio=0.66. When n is lowered to 4, then Ho=125 Oe and the balanceratio=0.99.

Referring to FIG. 11, the viability of a SAF reference layer accordingto the second embodiment wherein the SAF reference layer has anAP2/Co/Ru/Co/CoFeB configuration is demonstrated by measuring the polarKerr signal which is approximately proportional to the perpendicularcomponent of magnetization. The horizontal axis on the plot is the field(H) that is applied perpendicular to the plane of the layers at amagnitude ranging from −18000 Oe to +18000 Oe. Note that in all of thesamples where CoFeB thickness varies from 7.5 Angstroms (curve 41) to12.5 Angstroms (curve 42), the Kerr signal changes sign abruptly forH=−500 Oe. At this field, the AP1 (CoFeB) magnetization flips over. Forthe CoFeB dusting layer to be perpendicularly magnetized, its thicknessmust be less than a threshold value which in this case is around 10Angstroms. When the CoFeB thickness in the SAF structure is <10Angstroms, the CoFeB layer has PMA and switches with the AP2 layer.After switching, the magnetization (PMA) is constant up to the spin floptransition field of about 5000 Oe (point 60 a or 60 b) where theantiferromagnetic field coupling between the AP2 and CoFeB layers isovercome. For a CoFeB layer thickness >10 Angstroms, the PMA impartedfrom the AP2 layer through the Ru coupling layer is not strong enough tosustain perpendicular magnetization in the CoFeB layer. The absence ofPMA in the thicker CoFeB layers is indicated by the steady increase ofthe Kerr signal due to the CoFeB layer when H>0 and the absence of spinflopping at higher fields.

Referring to FIG. 12, a similar MTJ stack to that described with regardto Example 1 was fabricated wherein the number of the laminates “n” inthe (Co2.5/Ni6)_(n) layer is varied from 3 to 10, and the Co₂₀Fe₆₀B₂₀layer thickness is kept at 7.5 Angstroms. Thus, the MTJ in this studyhas a bottom spin valve configuration represented byTaN20/Mg7/NiCr50/(Co2.5/Ni6)_(n)/Co4/Ru4/Co4/Co₂₀Fe₆₀B₂₀7.5/MgO(8/4ROX)/Co₂₀Fe₆₀B₂₀3/Ta20/Ru50wherein both of the (Co/Ni)_(n) AP2 layer and CoFeB layer in the SAFreference layer have PMA. Once again, a plot of Kerr signal vs. appliedfield (H) is illustrated. As the number of laminates “n” decreases from10 to 3, the Kerr signal diminishes and SAF coercivity (Hc) increases.The increase in Hc from 500 Oe for n=10 to 1000 Oe for n=3 is becausethe Mst balance ratio for a “n” value of 3 or 4 is approximately 1.00.An important finding is that SAF Hc is retained and even increased asthe AP2 layer becomes thinner. As a result, the requirement that SAFHc>free layer Hc is satisfied in this embodiment. Moreover, we havefound there is design latitude in terms of AP2 thickness and CoFeBthickness when fabricating a SAF structure for minimal fringing fieldHo.

Referring to FIG. 13 a, a plot is provided that shows change in loopshift as a function of junction resistance for a MTJ previouslyfabricated by the inventors that has aNiCr/(Co/Ni)₁₀/Co4/Ru4/Co4/(Co/Ni)₆/Ta1.5/CoFeB6/Co4/MgO/CoFeB12/Taconfiguration where NiCr is a seed layer, a 1.5 Angstrom thick Ta layeris inserted between the (Co/Ni)₁₀/Co/Ru/Co/(Co/Ni)₆ reference layer andCoFeB/Co transition layer, MgO is a tunnel barrier layer, CoFeB is thefree layer, and the uppermost Ta layer is a cap layer. Note there is ahigher resistance (Rp) as the physical size of the patterned MTJ becomessmaller. Thus, the devices in column 61 have a nominal size of 480 nmwhile the devices in columns 62-66 have a nominal size of 365 nm, 280nm, 200 nm, 145 nm, and 110 nm, respectively. It should be understoodthat the actual physical size of each device after the fabrication stepis typically 10 to 30 nm smaller than the corresponding nominal size dueto shrinkage of features as a result of reactive ion etching, forexample. Each data point represents one device and the measurement istaken across the wafer. To obtain the loop shift, resistance is measuredas a function of applied field. The resulting curve (not shown) is ahysteretic loop as a function of field and the reported loop shift Ho isthe field corresponding to the hysteretic loop center.

Referring to FIG. 13 b, a similar plot to that of the reference in FIG.13 a is shown except the MTJ is formed according to an embodiment of thepresent disclosure. In this example, the MTJ has aNiCr/(Co/Ni)₄/Co4/Ru4/Co4/Ta1.5/CoFeB6/MgO/CoFeB12/Ta configuration thatis fabricated according to an embodiment corresponding to FIG. 10. Thecomposite reference layer has (Co/Ni)₄ as the AP2 layer, two Co dustinglayers, a Ru anti-ferromagnetic coupling layer, a 1.5 Angstrom thick Tainsertion layer, and an upper CoFeB layer in the SAF structure. Eachdevice size has a Ho value substantially less than the result in FIG. 13a and demonstrates the effectiveness of the SAF structure of the presentdisclosure in meeting the objective of a lower Ho value. For example,data points 71 (FIG. 13 b) corresponding to the largest nominal devicesize of 480 nm are at lower Ho values than data points 61 for the samedevice size in FIG. 13 a. Likewise, data points 76 corresponding to thesmallest nominal device size of 110 nm are at lower Ho values than datapoints 66 for the same device size in FIG. 13 a. Again, physical size isabout 10 to 30 nm smaller for each device represented in the FIG. 13data plot. In addition to reduced Ho values at each feature size in FIG.13 b, coercivity in the (Co/Ni)₄ AP2 layer is increased compared withthe thicker (Co/Ni)₁₀AP2 layer in the reference sample in FIG. 13 a.

In the exemplary embodiments, the CoFeB composition in the SAF structurehas been set at Co₂₀Fe₆₀B₂₀. It should be understood that as the Fecontent is increased to Co₁₀Fe₇₀B₂₀, for example, the Mst contributionfrom the CoFeB layer will increase for a given thickness. Furthermore,Hc and Ho will increase as the Fe content becomes larger. On the otherhand, as B content becomes greater by replacing a Co₂₀Fe₆₀B₂₀ layer witha Co₂₀Fe₅₀B₃₀ layer, for example, the Mst contribution from the CoFeBlayer will decrease, and Hc and Ho will decrease as well. Therefore,additional flexibility in adjusting Ho is realized by modifying theCoFeB composition in the SAF structure. Preferably, the Fe content inthe CoFeB layer is greater than 20 atomic % and the B content is from 10to 40 atomic %.

With regard to a process of forming the various spin valveconfigurations of the aforementioned embodiments, all of the layers inthe MTJ stacks may be laid down in a sputter deposition system. Forinstance, a MTJ stack of layers may be formed in an Anelva C-7100 thinfilm sputtering system or the like which typically includes threephysical vapor deposition (PVD) chambers each having 5 targets, anoxidation chamber, and a sputter etching chamber. At least one of thePVD chambers is capable of co-sputtering. Typically, the sputterdeposition process involves an argon sputter gas with ultra-high vacuumand the targets are made of metal or alloys to be deposited on asubstrate. All of the CPP layers may be formed after a single pump downof the sputter system to enhance throughput.

As mentioned earlier, the MTJ stacks may be annealed by applying atemperature from 200° C. up to 350° C. or in the preferred embodimentsup to 500° C. for a period of 5 minutes to 10 hours. No applied magneticfield is necessary during the annealing step because PMA is establisheddue to the (111) texture in the composite seed layer 21 and due to theCo—Ni (or A1-A2) spin orbital interactions in the laminated referencelayer 22 or laminated FL2 free layer 31. Thereafter, a conventionalphotoresist patterning and etching sequence may be performed tofabricate the MTJ stack of layers into a plurality of islands(nanopillars) having a circular, elliptical, or rectangular shape from atop-down view. Next, an insulation layer (not shown) may be deposited onthe substrate 20 followed by a planarization process to make theinsulation layer coplanar with the cap layer 27 on each MTJ nanopillar.Finally, a top electrode (not shown) may be formed on the cap layer asappreciated by those skilled in the art.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

We claim:
 1. A synthetic antiferromagnetic (SAF) structure, comprising:(a) a FL2 free layer with intrinsic perpendicular magnetic anisotropy;(b) a CoFeB layer wherein perpendicular magnetic anisotropy isestablished by antiferromagnetic coupling with the FL2 free layerthrough an antiferromagnetic (AF) coupling layer formed between the FL2free layer and CoFeB layer; and (c) the AF coupling layer made of anon-magnetic material to give an FL2/AF coupling/CoFeB configuration oran CoFeB/AF coupling/FL2 configuration.
 2. The SAF structure of claim 1wherein the FL2 layer is comprised of an (A1/A2)_(n) laminate where thelamination number “n” is less than 6, A1 is one of Co, CoFe, or an alloythereof, and A2 is one of Pt, Pd, Rh, Ru, Ir, Mg, Mo, Os, Si, V, Ni,NiCo, and NiFe, or A1 is Fe and A2 is V.
 3. The SAF structure of claim 1wherein the FL2 layer is a L10 ordered alloy of the form MT wherein M isRh, Pd, Pt, Ir, or an alloy thereof, and T is Fe, Co, Ni or alloythereof, or the FL2 layer is made of a rare earth-transition metal alloythat is TbCo, TbFeCo, or GdFeCo.
 4. The SAF structure of claim 1 whereinthe FL2 layer is comprised of a (A1/A2)_(n) laminate where n is thelamination number, A1 is one of Co, CoFe, or an alloy thereof, A2 is oneof Rh, Ir, Ru, Os, Mo, or an alloy thereof and A2 provides ferromagneticor antiferromagnetic coupling between neighboring A1 layers.
 5. The SAFstructure of claim 1 wherein the CoFeB layer has a thickness from about6 to 12 Angstroms.
 6. The SAF structure of claim 1 wherein the AFcoupling layer is Ru, Ir, Rh, Os, Mo, V, or an alloy thereof.
 7. The SAFstructure of claim 1 wherein the SAF structure serves as a free layer ina magnetic tunnel junction (MTJ) and has the FL2/AF coupling/CoFeBconfiguration in a top spin valve MTJ, or the CoFeB/AF coupling/FL2configuration in a bottom spin valve MTJ.
 8. The SAF structure of claim1 wherein the SAF structure is a free layer in a top spin valve MTJ andis formed on a seed layer made of TaN/Mg/X, Ta/X, or Ta/Mg/X where X isa growth enhancement layer made of NiCr or NiFeCr.
 9. The SAF structureof claim 8 wherein the MTJ is further comprised of a MgO tunnel barrier,reference layer, and cap layer that are sequentially formed on a topsurface of the CoFeB layer, and the MgO tunnel barrier inducesadditional PMA in the CoFeB layer.
 10. The SAF structure of claim 9wherein the reference layer has perpendicular magnetic anisotropy. 11.The SAF structure of claim 1 further comprised of a first Co or Co alloydusting layer formed between the AF coupling layer and the FL2 freelayer, and a second Co or Co alloy dusting layer formed between the AFcoupling layer and CoFeB layer.
 12. A method of fabricating a syntheticantiferromagnetic (SAF) structure, comprising: (a) providing a seedlayer on a substrate; (b) depositing a ferromagnetic layer havingintrinsic perpendicular magnetic anisotropy (PMA) on the seed layer; (c)depositing an antiferromagnetic (AF) coupling layer on a top surface ofthe ferromagnetic layer; and (d) depositing a CoFeB layer on a topsurface of the AF coupling layer wherein perpendicular magneticanisotropy is induced in the CoFeB layer through AF coupling with theferromagnetic layer.
 13. The method of claim 12 wherein the seed layeris TaN/Mg/X, Ta/X, or Ta/Mg/X where X is NiCr or NiFeCr, or the seedlayer has a Ta/M1/M2 composition where M1 is Ru, and M2 is one of Cu,Ti, Pd, Pt, W, Rh, Au, or Ag.
 14. The method of claim 12 wherein theferromagnetic layer is an AP2 reference layer comprised of an(A1/A2)_(n) laminate where the lamination number “n” is less than 6, A1is one of Co, CoFe, or an alloy thereof, and A2 is one of Pt, Pd, Rh,Ru, Ir, Mg, Mo, Os, Si, V, Ni, NiCo, and NiFe, or A1 is Fe and A2 is V,or the AP2 reference layer is made of a L10 ordered alloy of the form MTwherein M is Rh, Pd, Pt, Ir, or an alloy thereof, and T is Fe, Co, Ni oralloy thereof, or the AP2 reference layer is made of a rareearth-transition metal alloy that is TbCo, TbFeCo, or GdFeCo.
 15. Themethod of claim 12 wherein the ferromagnetic layer is a FL2 free layercomprised of an (A1/A2)_(n) laminate where the lamination number “n” isless than 6, A1 is one of Co, CoFe, or an alloy thereof, and A2 is oneof Pt, Pd, Rh, Ru, Ir, Mg, Mo, Os, Si, V, Ni, NiCo, and NiFe, or A1 isFe and A2 is V, or the AP2 reference layer is made of a L10 orderedalloy of the form MT wherein M is Rh, Pd, Pt, Ir, or an alloy thereof,and T is Fe, Co, Ni or alloy thereof, or the AP2 reference layer is madeof a rare earth-transition metal alloy that is TbCo, TbFeCo, or GdFeCo.16. The method of claim 12 wherein the CoFeB layer has a thickness fromabout 6 to 12 Angstroms.
 17. The method of claim 14 further comprised offorming a first Co or Co alloy dusting layer between the ferromagneticlayer and AF coupling layer, and forming a second Co or Co alloy dustinglayer between the AF coupling layer and CoFeB layer to improve the AFcoupling between the ferromagnetic layer and CoFeB layer.
 18. The methodof claim 17 further comprised of sequentially forming a tunnel barrierlayer made of a metal oxide, a free layer with PMA, and a cap layer on atop surface of the CoFeB layer to form a MTJ stack of layers, the tunnelbarrier induces additional PMA in the CoFeB layer.
 19. The method ofclaim 18 further comprised of annealing the MTJ stack with a temperaturebetween about 200° C. and 500° C. for a period of about 5 minutes to 10hours.